Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to a projection cathode
ray tube for projecting the image displayed on a fluorescent
surface on a screen in front of the fluorescent surface
through a projcction lens as an enlarged image. More
particularly, the present invention relates to a projection
cathode ray tube which is capable of reducing the deteriora-
tion of the light output with time by suppressing the
browning phenomenon of the glass surface of the face panel
and the multilayer optical interference film.
~RIEF DESCRIPTION OF T~E DRAWINGS
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Fig. 1 is a sectional view of an embodiment of a
projection cathode ray tube provided with a multilayer
optical interference film according to the present inven-
tion;
Fig. 2 shows the deterioration of the light output of a
projection cathode ray tube with time;
Fig. 3 shows a change in the spectral transmittance due
to browning on the glass surface of the face panel; and
Fig. 4 is a sectional view of a conventional projection
cathode ray tube provided with a multilayer optical inter-
ference film.
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Description of the Related Art
In USP No. 4,642,695 filed by the applicant of thepresent invention is disclosed a method of ameliorating the
defect of a projection television set, namély, the poor
convergence ratio exhibited when the beams of the respective
monochromes emitted from the projection cathode ray tube are
received by the projection lens unit.
In an ordinary cathode ray tube, the light emitted from
the fluorescent surface assumes a state approximate to what
is called perfect diffusion light, but in a projection
television set, among the beams emitted from the fluorescent
surface, only the rays having a divergence angle of not more
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than about + 30 are received by the projection lens unit
and the other beams are treated as extraneous light. The
extraneous light is not only necessary but exerts various
deleterious influences. For example, the extraneous light
is reflected by a cylindrical mirror of the projection lens
unit or the like to become backlight, which lowers the
contrast of the projected image. According to the related
art disclosed in USP No. 4,642,695, the method is greatly
effective for improving the brightness of the image on the
screen of a projection television set because not less than
3096 of the total light fluxes emitted from a light emitting
point of the fluorescent surface is converged into the
interior of a conical body having a divergence angle of +
3oo~
In Japanese Patent Laid-Open No. 257043/1985 filed to
the Japan Patent Office by the applicant of the present
invention, a projection cathode ray tube provlded with a
multilayer optical interference film composed of a plurality
of alternately superimposed layers of high refractive and
low refractive index materials disposed between the face
panel and the fluoreecent surface is disclosed as the
concrete example of the above-deecribed related art. As an
example of the multilayer optical lnterference film, a
multilayer optical interference film ie described which is
compoeed of eix alternately euperimposed layere of tantalum
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pentoxide (Ta2o5) as a high refractive iindex material and
silicon dioxide (sio2) as a low-refractive index material.
In a conventional projection cathode ray tube provided
with a multilayer optical interference film on the inner
surface of the face panel, the degree to which the light
emitted from the projection cathode ray tube is lowered with
the operation time is disadvantageously larger than in a
projection cathode ray tube having no optical multilayer
interference film. Fig. 2 shows a change in the light
output with respect to the operation time which is obtained
by continuously operating a projection cathode ray tube
emitting green light (G) at a high voltage (accelerating
voltage) of 32 KV and a current density on the fluorescent
surface of 6 ~A ~ cm 2 (the outer surface of the face panel
of the projection cathode ray tube is cooled by a coolant).
In Fig. 2, the curve ~I) shows the deterioration of the
light output of a conventional projection cathode ray tube
which has no multilayer optical interference film. It i9
observed that the light output is lowered to 74~ of the
initial light output in 7,000 hours. This deterioration
will be ascribed to the fact that the luminous efficiency of
the phosphor itself is lowered and to the browning phenome-
non of the face panel. The ratio of the weights of these
causes is con~idered to be about 50~ in the present state of
art.
It is considered that the luminous efficiency of a
phosphor is lowered when the luminescent mechanism of the
phosphor itself is gradually broken by the energy of the
impact of the electron beam and the heat or the X-rays
generated thereby. The browning phenomenon is divided into
electron beam browning and X-ray browning. Electron beam
browning is caused by the reduction of alkaline metal ions
such as sodium (Na) ions and potassium (K) ions which
constitute the face panel into metals by the energy produced
when the electron beam which has passed through the gaps of
the fluorescent layer directly collides against the inner
surface of the face panel. X-ray browning is a kind of
solarization and is caused when the energy of the X-rays
produced by the electrons which collide against the fluores-
cent surface or the glacs surface at a high speed produces
the browning center in the lattice defect in the glass
eurface of the face panel. If such electron beam browning
or X-ray browning is caused, the glass surface of the face
panel iq tinged with brown and the spectral transmittance is
lowered, as shown in the spectral transmittance distribution
(b) in comparison with the spectral transmittance distribu-
tion ~a) before browning in Fig. 3. The lowering of the
tran~mittance becomes larger in the short wavelength range
of the visible light.
3 ~ 3
The curve (II) in Fig. 2 shows the deterioration of the
light output of a conventional projection cathode ray tube
having a multilayer optical interference film which is
composed of a face panel 1, a multilayer optical interfer-
ence film 2 provided on the inner surface of the face panel
and consisting of five alternately superimposed layers of
titanium oxide ~TiO2) as a high refractive index material
and silicon dioxide (SiO2) as a low refractive index
material, a phosphor layer 3 and a metal back coat 4
overlaid with each other on the optical multilayer
interference film, as shown in the sectional view of the
face panel and the fluorescent surface of a projection
cathode ray tube of Fig. 4. It is observed that the light
output is lowered to 63% of the initial light output in
7,000 hours. The deterioration of the light output is much
larger than that in the conventional projection cathode ray
tube having no multilayer optical interference film (curve
(I)). As a result of the analysis of the cause of the
deterioration, it has been found that browning is produced
on the multilayer optical interference film 2 in addition to
the glass surface of the face panel 1. Browning on the
multilayer optical interference film 2 is produced on, in
particular, the layer of titanium oxide ~TiO2), which is a
high refractive index material. It ha~ been found that
browning on the titanium oxide layer i~ caused by the
reduction of Tio2 into TiO2 x by the energy produced when
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the electron beam having a high energy which has passed
through the gaps of the phosphor layer 3 rushes into the
titanium oxide (TiO2) layer. As a high refractive index
material, an oxide of a me~al is ordinarily used. As a
result of investigations of various metals which are
optically usable, it has been confirmed that a similar
browning phenomenon is caused to one degree or another by
using any material.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention
to eliminate the above-described problems in the related art
and to provide a projection cathode ray tube provided with a
multilayer optical interference film which is capable of
reducing the deterioration of the light output with time by
suppressing the browning phenomenon of the face panel and
the multilayer optical interference film.
To achieve this aim, in a projection cathode ray tube
according to the pre~ent invention, the outermost surface of
a multilayer optical interference film consisting of optical
thln film layers of alternately superimposed high refractive
and low refractive index materials and provided between the
fluorescent surface and the face panel is coated with a
transparent fllm of an inorganic material such as silicon
dioxide which is optically transparent and ~table with
respect to the lmpact of an electron beam. The thickness of
the transparent film is not less than 1.0 ~m.
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In a projection cathode ray tube according to the
present invention, since a protective film of an inorganic
material which is optically transparent and stable is formed
on the outermost ~urface of the multilayer optical interfer-
ence film so as to protect the multilayer optical interfer-
ence film from the impact of an electron beam, even an
electron beam having a high energy which has passed through
the gaps of the phosphor layer loses the energy in the
protective film. It is therefore possible to reduce the
browning on the multilayer optical interference film and the
glass surface of the face panel.
The above and other objects, features and advantages of
the present invention will become clear from the following
description of the preferred embodiment thereof, taken in
conjunction with the accompanying drawings.
DESCRIPTION OF ~HE PREFERRED EMBODIMENT
An embodiment of the present invention will be ex-
plained hereinunder.
Fig. 1 is a sectional view of the face panel and the
fluorescent surface of an embodiment of a projection cathode
ray tube provided with a multilayer optical interference
film according to the present invention.
On the inner surface of the face panel 1 are provided a
multilayer optical interference film 2 consisting of five
alternately superimpo5ed layers of titanium oxide (TiO2) as
a high refractive index material and silicon dioxide (SiO2)
as a low refractive index material in the same way as in the
re~ated art. In the present invention, the outermost
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surface of the multilayer optical interference film Z is
coated with a transparent film 5 of an inorganic material.
A phosphor layer 3 and a metal back coat 4 are provided on
the transparent film 5 in the same way as in the related
art. It is necessary that the transparent film 5 of an
inorganic material not only absorbs the energy of an
electron beam having a high energy as much as possible which
has passed through the gaps of the phosphor layer 3 but also
transmits the light emitted from the phosphor layer 3 with
as little
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loss as possible. It is also necessary that the transparent
film 5 of an inorganic material is optically transparent
with respect to the multilayer optical interference film 2
provided therebeneath, so that there is a possibility of
limiting the refractive index or the film thickness of the
transparent film 6 of an inorganic material. It goes
without saying that the transparent film 5 of an inorganic
material is required to be stable with respect to the impact
of an electron beam. A projection cathode ray tube provided
with a multilayer optical interference film using a silicon
dioxide ~SiO2) film of 5.0 ~m thick as the transparent film
S of an inorganic material was produced on an experimental
basis. The projection cathode ray tube was continuously
operated at a high voltage (accelerating voltage) of 32 KV
and a current density on the fluorescent surface of 6 ~A -
cm 2 in the same way as in the related art. A change in
the light output with the operation time in this case is
~hown by the curve (III) in Fig. 2. In this case, due to
the electron beam energy absorbing effect of the transparent
film (5) of an inorganic material, the browning phenomenon
on the multilayer optical interference film 2 and the glass
surface of the face panel 1 was suppressed. The deteriora-
tion of the light output was 81% of the initial light output
in 7,000 hours. Thi~ is rather smaller than the deteriora-
tion ~74~ of the initial light output) of the light output
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of the conventional projection cathode ray tube having no
optical multilayer interference film. As the inorganic
material for the transparent film 5, various materials other
than SiO2 may be used such as the oxides, fluorides and
sulfides of inorganic elements. The necessary film thick-
ness of the transparent film (5) of an inorganic material
varies depending upon the property of the material used.
The depth d to which an electron beam enters a sub-
stance is represented by the well known equation:
d = 2.5 x 10-12p-1V2 (cm)
wherein p is the density of the substance and V is the
accelerating voltage of the electron beam.
In the case of silicon dioxide ~SiO2), the high voltage
(accelerating voltage) is 32 XV and the depth to which the
electron beam enters silicon dioxide (SiO2) is about 10 ~m,
but since the energy of the electron beam is rapidly lost in
comparison with the depth to which the electron beam enters,
the film thickness of 10 ~m is unnecessary. When the film
thickness was not less than 1.0 ~m, the browning reducing
effect wa~ observed, and when the film thickness was 5.0 ~m,
approximately sufficient effect was exerted. With the use
of materials other than silicon dioxlde (SiO2), approximate-
ly the same effect was obtained.
As described above, according to the present invention,
slnce the outermost surface of the multilayer optical
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interference film of a projection cathode ray tube is coated
with a transparent film of an inorganic material which is
stable with respect to the impact of an electron beam, the
energy of the electron beam is lost in this protective film,
and browning on the multilayer optical interference film and
the glass surface of the face panel is reduced. Thus, it is
possible to provide a high-quality projection cathode ray
tube which is capable of reducing the deterioration of the
light output with time.
While there has been described what is at present
considered to be a preferred embodiment of the invention, it
will be understood that various modifications may be made
thereto, and it is intended that the appended claims cover
all Cuch modifications as fall within the true spirit and
scope of the invention.
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